1. Field of the Invention
The present invention relates to a tunable wavelength four light wave mixer for converting the wavelength of a signal light.
2. Description of the Related Art
A light wavelength multiplex transmission system is finding applications in which a plurality of signal light of different wavelengths are multiplexed and transmitted through a single optical fiber. In this wavelength multiplex transmission system, it is desired to exchange information between different wavelengths and to convert the wavelength of the light signal without photoelectric conversion.
With the increase in the band of light amplifiers, a network having a different wavelength band is expected to appear. In such a case, wavelength conversion without photoelectric conversion is desired. Further, the long-distance transmission of light signals poses the problem of waveform distortion generated by wavelength dispersion and the nonlinear optical effect of the optical fiber. A light signal with wavelength distortion is converted to phase conjugate light and propagated through a transmission path having the same amount of dispersion and nonlinearity, thereby making it possible to restore the light signal of the original wavelength.
The configuration of a four light wave mixer using an optical fiber, a semiconductor light amplifier, a distributed optical feedback laser diode or the like is known. A known conventional configuration uses an optical fiber as shown in FIG. 1. In FIG. 1, reference numeral 101 designates a dispersion-shifted optical fiber (DSF) of zero dispersion wavelength xcex0, numeral 102 a pump light source (LD) for generating the pump light of wavelength xcexp, numeral 103 an optical bandpass filter (BPF) for passing the band of wavelength xcexc, numeral 104 an optical fiber for inputting the signal light, numeral 105 a wavelength multiplexing unit for multiplexing the signal light of wavelength xcexs, with the pump light of wavelength xcexp and applying the resulting light into the dispersion-shifted optical fiber 101, and numeral 106 an optical fiber for outputting the converted light.
Assume that the zero dispersion wavelength xcex0 of the dispersion-shifted optical fiber 101 is equalized with the wavelength xcexp of the pump light from the pump light source 102 and that the signal light of wavelength xcexs and the pump light of wavelength xcexp are applied to the dispersion-shifted optical fiber 101 through the multiplexing unit 105. Then, the light of wavelength xcexs, xcexp containing the converted light of wavelength xcexc(≈2xcexpxe2x88x92xcexs) symmetric about the pump light wavelength xcexp with respect to the signal light wavelength xcexs is output, and the converted light of wavelength xcexc can be output by the optical bandpass filter 103. The converted light thus obtained by the four light waves mixing has an intensity proportional to that of the incident signal light and also has a phase proportional to the complex conjugate of the signal light.
For improving the efficiency of four light waves mixing using the optical fiber, the phase matching conditions must be satisfied by rendering the pump light wavelength coincident with the zero dispersion wavelength of the optical fiber. In order to compensate for the variations along the longitudinal direction of the zero dispersion wavelength of the optical fiber, an optical fiber of a predetermined length is segmented and the segmented optical fibers are combined and connected so that a reference zero dispersion wavelength is secured as a whole due to the length of each segmented optical fiber and the difference between the average zero dispersion wavelength and the reference zero dispersion wavelength (See Japanese Unexamined Patent Publication No. 7-84289, for example).
Since the zero dispersion wavelength is unique to the optical fiber, however, the wavelength of the converted light is determined naturally by determining the signal light wavelength, and therefore the wavelength of the converted light lacks the freedom. The probable next choice is to change the wavelength of the pump light. In that case, the wavelength of the pump light is displaced from the zero dispersion wavelength, and the phase matching conditions can no longer be satisfied due to the difference in propagation rate between the pump light and the signal light caused by the dispersion characteristic. Thus, the conversion efficiency is deteriorated, with the result that only a very small change in the wavelength is possible.
The object of the present invention is to increase the freedom of the wavelength of the converted light.
According to a first aspect of the invention, there is provided a tunable wavelength four light wave mixer in which the signal light and the pump light having the same wavelength as the zero dispersion wavelength of a dispersion-shifted optical fiber are applied to the dispersion-shifted optical fiber thereby to output the light having a converted signal light wavelength, wherein the dispersion-shifted optical fiber is composed of a plurality of sequentially connected dispersion-shifted optical fibers having different zero dispersion wavelengths xcex01, xcex02, the mixer having a pump light source for applying the pump light of the same wavelength xcexp of as any one of the different zero dispersion wavelengths and the signal light of the wavelength xcexs. As a result, the converted light can be output from the optical bandpass filter with the wavelength of xcexc1 or xcexc2.
According to a second aspect of the invention, there is provided a tunable wavelength four light wave mixer further comprising a plurality of pump light sources for outputting the pump light of wavelengths xcexp1, xcexp2 corresponding to a plurality of dispersion-shifted optical fibers of different zero dispersion wavelengths or a pump light source capable of selectively outputting the pump light of wavelength xcexp1, xcexp2.
According to a third aspect of the invention, there is provided a tunable wavelength four light wave mixer further comprising a variable attenuator, a light amplifier, a pump light source for controlling the output power, etc. to equalize the input power of the pump light having the same wavelength as the zero dispersion wavelength for each section of a plurality of dispersion-shifted optical fibers of different zero dispersion wavelengths.
According to a fourth aspect of the invention, there is provided a tunable wavelength four light wave mixer further comprising an oscillator for avoiding the restriction of the pump light input power due to the stimulated Brillouin scattering by frequency-modulating or phase-modulating the pump light.
According to a fifth aspect of the invention, there is provided a tunable wavelength four light wave mixer in which the high nonlinear optical fiber with a smaller core diameter can be used as a dispersion-shifted optical fiber.
According to a sixth aspect of the invention, there is provided a tunable wavelength four light wave mixer further comprising a detection means for detecting the converted light and a control unit for controlling the output power of the pump light source, the pump light input power to the dispersion-shifted optical fiber and the central wavelength of the pass band of the variable optical bandpass filter in such a manner as to secure the same output power of the converted light.